1. Field of the Invention
This invention relates to a method of effectively erasing a residual image on a stimulable phosphor sheet which is used repeatedly, and an apparatus for carrying out the method. This invention particularly relates to a method of and apparatus for effectively erasing a residual image on the stimulable phosphor sheet repeatedly used in a radiation image recording and reproducing system wherein the stimulable phosphor sheet is exposed to a radiation to have a radiation image stored therein and then exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy, the emitted light is detected and converted into an electric image signal, and the electric image signal is processed and used for reproducing a visible image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Thus, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in proportion to the radiation energy stored. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy. The finally obtained visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). The stimulable phosphor sheet used in the radiation image recording and reproducing system may be in any of various forms such as a panel, drum or the like, which are herein generally referred to as sheets. For economical reasons, it is desirable that the stimulable phosphor sheet be used repeatedly.
Theoretically, when the stimulable phosphor sheet is exposed to stimulating rays of a sufficient intensity to cause the sheet to emit light in proportion to the stored radiation energy at the image read-out step, the stored radiation energy should be released completely and disappear. Actually, however, since the intensity of the stimulating rays used at the image read-out step is not sufficient to completely erase the stored radiation energy, a part of the stored radiation energy remains unerased in the stimulable phosphor sheet so that when the sheet is used repeatedly, the unerased radiation energy causes noise to arise in the visible image reproduced from the reused sheet.
The inventors conducted experiments to find what level of radiation energy of the residual image on the reused stimulable phosphor sheet causes noise to arise in the reproduced visible image to an extent adversely affecting the image viewing, particularly diagnosis. From the results of these experiments, it has been found that, in order to eliminate the detrimental noise due to the residual image, the radiation energy of the radiation image stored in the stimulable phosphor sheet must be erased to the order of 10.sup.-4 to 10.sup.-6. Stated differently, the original radiation energy stored in the sheet must be erased to a level between 0.01 and 0.0001 when the maximum of the original level is 100.
As described, for example, in U.S. Pat. No. 4,400,619, the aforesaid residual image can be erased by stimulating the stimulable phosphor sheet by light having a wavelength within the stimulation wavelength range for the stimulable phosphor constituting the stimulable phosphor sheet before the next image recording on the stimulable phosphor sheet, thereby sufficiently releasing the stored radiation energy. In order to completely erase the residual image, the light exposure amount (i.e. illuminance.times.time) should be adjusted to be as large as possible. However, in order to erase the radiation energy of the previously stored radiation image to the order of 10.sup.-4 to 10.sup.-6, the stimulable phosphor sheet must be exposed to a high illuminance for a long period, for example to 30,000 lx for 100 to 1,000 seconds by use of a tungsten-filament lamp or the like. Therefore, from the viewpoint of decrease in energy requirement and erasing time, and service life of the erasing light source, it is desirable that the light exposure amount be limited to the minimum necessary value.
In order to satisfy the two incompatible requirements described above, the applicant in Japanese Unexamined Patent Publication No. 58(1983)-80633 (U.S. patent application No. 440,046, now U.S. Pat. No. 4,584,482), proposed detecting the residual radiation energy level stored in a stimulable phosphor sheet and adjusting the erasing light exposure amount in accordance with the detected residual radiation energy level. Since the level of the radiation energy remaining on the stimulable phosphor sheet is correlated with the level of energy of light emission from the sheet at the image read-out step, it is possible to detect the level of the residual radiation energy of the basis of the read-out signal, i.e. the output of an image read-out photodetector. Or, a photodetector of the same type as the image read-out photodetector may be used for detecting the level of residual radiation energy, and the level of residual radiation energy may be detected on the basis of the output of the photodetector.
In the case of a radiation image of the human body or the like, since the level of residual radiation energy is different among various portions of the image, the erasing light exposure amount is generally controlled so that the highest level of residual radiation energy can be released. Also, the image portion at which the level of residual radiation energy is highest is a solid image portion exposed to the largest amount of radiation at the image recording step. When a photomultiplier is used as the image read-out photodetector and light emitted by the solid image portion of the stimulable phosphor sheet is detected by use of the photomultiplier, since the image read-out gain is adjusted to suit the image read-out, the output current of the photomultiplier goes to the saturated condition. Or, the output current approaches the saturated condition, and the increase thereof with respect to an increase in the incident light amount gradually becomes small. In this case, the level of residual radiation energy at the solid image portion cannot be measured accurately. On the other hand, when an independent photomultiplier is specially used for detecting the level of residual radiation energy, since the gain of the photomultiplier can be adjusted to suit detection of the level of residual radiation energy at the solid image portion independently of image read-out, it is possible to measure the level of residual radiation energy at the solid image portion. However, when the photodetector is independently used for detecting the level of residual radiation energy, the size and the cost of the apparatus increase.